The present invention relates to a method of manufacturing a print head for use with an ink jet printer.
One prior art print head for use with an ink jet printer is of a piezoelectric type in which a drop of ink is ejected out by an increased pressure in an ink pressure chamber developed when the piezoelectric element is deformed. To make the print head, a piezoelectric element having two layers is formed with a plurality of grooves therein, each of which serves as an ink pressure chamber. Then, an adhesive is applied by screen printing to the top surfaces of the walls bounding the pressure chambers, and then a cover is placed on the surfaces of the walls. The cover closes the respective grooves to form ink pressure chambers.
This type of ink pressure chamber operates as follows:
As shown in FIG. 9A, each ink pressure chamber is defined by adjacent two walls 3a which are formed when a plurality of grooves are formed in the two-layer piezoelectric element 2. The chamber 1 is closed at its front by a nozzle plate 4. The nozzle plate 4 has a nozzle hole 6 through which ink 5 is ejected out from the chamber 1. The piezoelectric element 2 is polarized in the lateral direction (indicated by arrow P) in FIG. 9A of the ink pressure chamber 1.
When printing, an electric field is applied in the direction perpendicular to the direction of the polarization. The applied electric field causes deformation of the walls, so that a very small amount of ink, i.e., an ink drop, is ejected out through the nozzle hole 6 from the ink pressure chamber 1.
Specifically, application of a voltage to the walls 3a as shown in FIG. 9B, causes an electric field to be developed in the direction of the arrows. The walls 3a are deformed to extend outwardly relative to the chamber 1, causing the volume of the chamber 1 to increase. The increased volume results in a decrease in pressure in the chamber 1. Thus, ink 5 is supplied to the chamber 1 from a main ink-supplying system, not shown, by the amount of increased volume. Then, the direction of the electric field is reversed as shown in FIG. 9C. The walls 3a are deformed to extend inwardly relative to the chamber 1, causing an increase in the pressure of the chamber 1. The increased pressure causes an ink drop to be ejected through the nozzle hole 6 in the nozzle plate 4.
Finally, the respective walls 3a regain their original positions as shown in FIG. 9D, so that the ink drop 5a is separated from the ink in the chamber and is ejected from the nozzle hole 6, with the ink drop adhering to a print medium, not shown, to form a dot.
In manufacturing the aforementioned piezoelectric type print head, a plurality of grooves are formed in the two-layer piezoelectric element 2 to define a plurality of walls 3a and 3b as shown in FIG. 10A. Then, a cover 9 is bonded to the top surfaces (hatched areas in FIG. 10A) of the walls 3 using an adhesive 8 as shown in FIG. 10B, thereby defining ink pressure chambers 1. The walls 3b at ends of the row of the walls are thicker than the rest of walls 3a so as to protect thin walls 3a from inadvertently exerted outside forces.
Screen printing is widely used to apply the adhesive 8 on the top surface of the walls 3a and 3b.
As shown in FIG. 11A, a screen mask 11 is set in position in proximity to the top surfaces of the walls 3a-3b. The screen mask 11 has a pattern 12 in which mesh-like openings are formed, and the adhesive 8 is squeezed through the mesh-like openings. A predetermined amount of adhesive 8 is placed on one end of the pattern 12 as shown in FIG. 11B.
As shown in FIG. 11C, a squeegee 13 of the screen printer is first positioned in contact with the mask surface beside the adhesive 8, and then the squeegee 13 is caused to slide along the mask surface in the direction shown by an arrow as shown in FIG. 11D. The squeegee 13 travels while urging the adhesive 8 against the screen mask 11, and therefore the adhesive 8 is squeezed through the mesh-like openings to the top surfaces of the walls 3a-3b as shown in FIG. 11D. FIG. 11E shows the adhesive applied to the walls.
In the aforementioned prior art method, the adhesive 8 is transferred to the top surfaces of the walls 3a-3b by moving the squeegee 13 longitudinally of the walls along the grooves. Although each wall receives adhesive evenly applied along its length, the thickness of layer of the adhesive varies from wall to wall as shown in FIG. 12 due to slight differences in pressure applied to the adhesive across the width of the squeegee, and to variations in the contact condition between the squeegee and the screen mask 12.
Variations in the thickness of the adhesive layer result in poor bonding effect between the walls 3a-3b and the cover 9, which in turn causes differences in the amount of deformation of walls 3a-3b during printing operation. This causes variations in the amount of the ink discharged from the nozzle hole 6, leading to poor print quality. Moreover, there is a possibility of ink leaking through a portion to which insufficient adhesive is applied.
In addition, the same thickness of layer of adhesive 8 is applied to the top surfaces of all the walls 3a-3b across the length thereof as shown in FIG. 13A, and therefore the two walls 3b at extreme ends of row of walls 3a-3b receive more adhesive than the rest of walls since the two extreme walls 3b are thicker than the others. The grooves defined by the two extreme end walls 3b may be filled with an extra amount of adhesive as shown in FIG. 13B when the cover 9 is pressed against the walls 3a-3b. The adhesive leaked to fill in the groove suppresses deformation of the walls defining the groove, substantially deteriorating the function of pressurizing the ink in the chamber.
The adhesive 8 takes the form of, for example, an electrically conductive epoxy adhesive which contains electrically conductive particles having a size of about eight microns. The excessive adhesive entering the chamber may contact the adjacent electrode, short-circuiting each other.
One prior art method suggests providing several dummy grooves beside the piezoelectric element 2 in order to accommodate an extra amount of adhesive 8 when the cover 9 is pressed against the top surfaces of the walls. This method necessitates formation of extra grooves which impose additional manufacturing cost but do not serve to discharge ink.